The present invention relates to an actuating drive for a plasticizing unit of an injection molding machine, and more particularly to an actuating drive with a spindle drive for pulsating actuation of a mold ejector to reduce the electrical energy consumption of the spindle drive.
International patent publication WO 97/34757 describes a spindle drive operating as an actuating drive for an injection molding machine. The actuating drive includes an electric motor and, for relieving the electric motor in the feed phase, an energy storage device in the form of a compression spring assembly arranged in parallel with the electric motor. The load on the compression spring assembly increases during the return stroke of the spindle drive, whereas the spring force continuously decreases during the spindle forward feed stroke in conformance with the characteristic spring curve. However, the relief provided by the energy storage device is significantly reduced in that the characteristic spring curve of the energy storage device does not conform to the force and speed requirements of a plasticizing drive which can vary significantly depending on the stroke and can therefore negatively effect the total energy consumption of the drive unit. If the spindle drive is used as an actuating drive for the plasticizing screw, then the spindle drive has to be held by the electric motor at the return stroke position during the time between mold filling and injection process, counteracting the then maximum forward feed force of the energy storage device. Moreover, the motor also has to supply the force difference required for building up the necessary counter-pressure in the initial phase of the mold filling process, i.e., in the region of the forward stroke position of the plasticizing screw, where the spring force is minimal. The same applies when a spindle drive of this type is used as an actuating drive for the entire plasticizing unit: while the plasticizing unit is stopped in the return stroke position, the spindle drive has to be held by the electric motor in a stationary state against the maximum spring force. The electric motor also has to generate the injection force of the injection nozzle on the sprue bush of the injection tool during the entire duration of the injection process when the plasticizing unit is in the forward feed position. As a result, the compression spring assembly reduces the power required by the electromotive spindle drive only in a limited way.
It would therefore be desirable and advantageous to provide an improved plasticizing unit of simple construction, which obviates prior art shortcomings and is able to specifically reduce the power requirements of the electromotive spindle drive.